This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Debevec, P.T.

Fox, B.D.

Harty, P.D.

Jenkins, D.A.

Jones, R.T.

Simon, D.A.

English

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CALIBRATION O F  THE LARGE SOLID ANGLE DETECTOR 
P.T. Debevec, P.D. Harty, R.T. Jones, D.A. Simon and B.D. Fox 
University of lllinois, Champaign, lllinois 61820 
D.A. Jenkins 
Virginia Polytechnic Institute and State University, Blacks b urg, Virginia 24 061 
For the accurate measurement of proton cross sections over a wide angular range it 
is necessary to know the angular acceptance and efficiency of the detector. These can be 
calculated from the known geometry, and the detection efficiency as a function of proton 
energy can be estimated using a Monte Carlo code. In order to check the accuracy of this 
treatment a measurement of a well-known cross section was required. The reaction chosen 
was ' H ( ~ , ~ ) ' H ,  for which the cross section is well known. 
The experiment was performed using a 45 MeV beam of protons, which was produced 
by stripping a H, beam that was also being used for a Cooler run. The flux of incident 
protons was measured using an 20-cm wide plastic scintillator paddle in the beam at the 
downstream end of the detector. The singles rate in this scintillator was recorded, as 
well as coincidences between this scintillator in adjacent beam cycles. In this way it was 
possible to calculate the number of times there was more than one proton in a beam cycle, 
and hence the flux could be determined. 
A schematic diagram of the detector is shown in Fig. 1. It consists of 3 concentric rings 
of grounded sense wires at radii of 12, 23 and 34 cm, enclosed in a cylindrical chamber 37 
cm in radius and 105 cm long. Each sense wire is surrounded by field wires at high negative 
potential. The chamber is filled with a gas mixture of 90% helium and 10% methane, which 
Figure 1. A schematic diagram of the Large Solid Angle Detector. 
103 
has been found to provide high efficiency and gain, as well as stable operation for high 
voltages of about -2 kV. The chamber is surrounded by 8 AE scintillators (0.32 cm thick) 
and 8 E scintillators (2.54 cm). An "OR" of the 8 AE counters formed the trigger for the 
experiment. The target consists of an aluminized nylon bag 20.3 cm in diameter and 2.4 m 
long, which was filled with hydrogen gas at atmospheric pressure. 
Preliminary analysis of the data using the calculated acceptance and efficiency has 
been completed. The measured cross section included a small contribution from elastic 
proton scattering off nitrogen and helium, since these were detected in the target gas in 
concentrations of 0.5% and 6% respectively. These impurities were detected using a Gas 
Chromatograph. The preliminary result shows agreement with previous measurements to 
within 3% over most of the angular range. The acceptance used to determine the cross 
section is expected to change once some small differences in scintillator positions are taken 
into account. The measured cross section is very sensitive to any small change in the 
acceptance, thus the comparison between the two will provide a good test of its accuracy. 
The final stage of analysis is in progress. 
In June 1989 the detector described above will be taken to the Saskatchewan Accel- 
erator Laboratory in Canada for proton measurements using a tagged photon beam. The 
reactions to be investigated are 2 ~ ( 7 , p ) n  and * H ~ ( ~ , ~ ) ~ H  up to about 100 MeV photon 
energy. 


Calibration of the Large Solid Angle Detector

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Calibration of the Large Solid Angle Detector

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